Field of the Invention
The present invention relates to a method for operating a combustion chamber. It also relates to the configuration of a burner for operating such a combustion chamber.
Discussion of Background
So-called silo combustion chambers are equipped with burners operating on the "lean premix principle". These so-called "dry low NO.sub.x " burners are operated in accordance with a switching operation mode in which the burners are subdivided into relatively large burner groups. These burners themselves can be installed and operated in both silo combustion chamber and annular combustion chambers. Using an annular combustion chamber as an example, a row of premixing burners of different sizes are arranged at the inlet end and in the peripheral direction. The large premixing burners, which are the main burners of the combustion chamber, and the small premixing burners, which are the pilot burners of the combustion chamber, are positioned with outlet ends on a front wall of the combustion chamber; the premixing burners are arranged alternately and at a uniform distance from one another. In the case of the silo combustion chambers, the premixing burners provided are arranged in honeycomb fashion at the top end of the combustion chamber and are subdivided into groups which usually consist of one piloting burner and a plurality of piloted burners.
The burners are put into operation individually or in groups as a function of the load. A fuel distribution system includes a switching operation which permits individual burner groups to be switched on or off. The switching operation has the disadvantage that the burner equivalence numbers, and therefore the NO.sub.x emissions, vary greatly. In the case considered, the groups of burners are generally relatively large, by analogy with the known "dry low NO.sub.x " technique. This is associated with the fact that an operating concept limited to a few groups offers advantages in terms of hardware and software complication. Various modes of operation can be proposed as a basis, such as one in which a valve position varies with the load, another in which the fuel distribution varies with the load or yet another in which the fuel allocation for each burner depends on the load ratio.
This procedure leads to the burner equivalence numbers, which are decisive for the NO.sub.x emissions, being subject to great fluctuations and, therefore, the NO.sub.x emissions also fluctuate strongly. In order to reduce these combustion fluctuations as to load varies, it would be conceivable to increase the number of groups. In the ideal case, this would lead to individual triggering for each burner. However, gas turbines of large output power require powerful combustion chambers with a correspondingly large number of burners, which in turn makes it necessary to install a large number of valves and supply conduits for the burners. The steps for switching the burners on or off would, in themselves, be minimal, but the number of valves and supply conduits would create substantial hardware and software complications. An additional factor in such a mode of operation is that it is difficult to deal with fluctuating ambient conditions so that, in the end, combustion fluctuations would still be expected, contrary to the objective of keeping the NO.sub.x emissions constant and low over the whole of the operation, and particularly at full load.
Summarizing, it may be stated that the following effects prevent the NO.sub.x emissions from being kept constant in the case of variable ambient conditions:
the proportion of the air quantity induced which is used for cooling the turbine increases or falls with the ambient temperature (pressure) and the combustion chamber air quantity falls or rises reciprocally. Despite constant temperature, this leads to variable flame temperatures and, therefore, to varying production of NO.sub.x ; and, PA1 the change to the combustion chamber pressure because of the ambient conditions leads directly to a change in the formation of NO.sub.x in the combustion chamber.